Method of preparing a single crystal for improved electron emission



United States Patent 3,432,917 METHOD OF PREPARING A SINGLE CRYSTAL FOR IMPROVED ELECTRON EMISSION Laurence 0. van Someren, Wayland, Mass., assignor t0 Thermo Electron Engineering Corporation, Waltham,

Mass., a corporation of Delaware No Drawing. Filed Mar. 6, 1967, Ser. No. 620,650

U.S. Cl. 29558 2 Claims Int. Cl. H011 7/66; B23p 13/04 ABSTRACT OF THE DISCLOSURE A method for preparing the surface of a single crystal for optimal electron emission when used with a thermionic converter comprising the steps of:

Slicing a crystal along a pre-determined axis to present a plane having the highest atomic density, abrading the surface mechanically, heating the surface to a temperature of between 2250 C. and 2600 C. for a period of one to ten hours in a vacuum and allowing the crystal to cool under the same controlled atmospheric condition.

This invention relates generally to thermionic energy conversion and in particular to a new and improved method of the preparation of the surface of a single crystal to improve the quantity of the electron emitted therefrom.

Thermionic energy converters operating on basic principles have been utilized in ever increasing uses in scientific and commercial fields. The majority of these devices comprise in their simplest form a chamber either under vacuum or containing a plasma, a collector, and an emitter, which when heated, emits electrons which pass through the plasma to the collector. Consequently, the most important unit of this converter is the emitter which is generally constructed of a metal such as tungsten, tantalum, or rhenium. Various of these base metals have been utilized; however, for practical purposes, tungsten and rhenium crystals have been shown to be among the best suited for use as emitters. Single crystals where there is a regular arrangement of atoms, are known to be more uniform thermionic emitters and can be chosen to present a particularly favorable plane for thermionic emission. Single crystals have the advantage in that a particular plane may be chosen which may be determined by X-rays to get a more uniform surface.

Therefore, an object of this invention is to provide a new and improved method of preparing a single crystal surface for use as an emitter to establish a capacity for greater emission current densities.

It is another object of this invention to provide a new and improved thermionic emitter which has greater uniformity of thermionic emission.

Other objects of this invention will in part be obvious and will in part appear hereinafter.

Crystals by their very nature have their constituent atoms arranged in regular three-dimensional patterns called space lattices. For purposes of electron emission, the atomic arrangement is extremely important for it has been determined that in alkali-metal plasma thermionic converters, the planes having the highest atomic density are most favorable for electron emission.

For purposes of this invention the single tungsten crystal is sliced to expose the (110) Miller plane parallel to the surface of the emitter. Similarly with rhenium the plane selected is that of the highest atomic density and is normally (0001) Miller.

The face of the crystal must then be smoothed to create a flat surface and to remove imperfections created by the slicing method. This is accomplished by grinding and it has been determined that a further abrading of the surface using five (5) micron particles of alumina applied as an aqueous slurry on a glass plate produces microscopic mechanical damage to the surface in the form of cracks, scratches and disturbed material. All traces of the alumina are removed from the crystal surface by successive washing and rinses in soapy water, clear water and tri-chloroethylene in an ultra-sonic bath.

The surface of the crystal is then heat-treated to stabilize its condition. Three different types of heat treatments may be used, radiant, RF induction, and electron bombardment, has been used due to the ease of control and regulation of the temperature. The crystal is placed in a vacuum of less than 10 torr and a temperature in the range between 2250 C. and 2600 C. is established for a period of between five to ten hours by the use of conventional electron bombardment equipment. The changes that take place on the surface of the crystal are in direct relationship to the time and temperature effected. It has been determined that the atoms in the abraded and damaged layer re-arrange themselves during the heat treat ment step so that the resulting surface displays the most favorable emission projection of the underlying single crystal material. The final step is to allow the crystal to cool under the controlled atmospheric state established during the heating stage.

It has been discovered that the crystal prepared in the above manner has a work function in the range of 5.0 to 5.1 electron volts. This is significantly higher than heretofore known methods of preparing useful single crystal surfaces or materials whose work function is in the range of 4.5 to 4.7 electron volts.

This increase in bare Work functionallows a very substantially increased electron emission current to be drawn from the emitter when operating in an alkali-metal plasma converter. The benefits obtained from the surface may be applied either to increase the saturation emission current from the emitter or to allow the converter to operate with a lower pressure of the alkali-metal in the plasma. In the latter case, the transport losses in the plasma are substantially reduced. The enhanced uniformity of the emitters prepared as described allows them to function with a greater efiiciency than resulting from methods heretofore utilized.

While there have been described herein what are at present considered to be preferred embodiments of the invention, it will be obvious to those skilled in the art that modifications and changes may be made without departing from the essence of this invention.

It is therefore to be understood that the exemplary embodiments are illustrative and not restrictive of the invention, the scope of which is defined in the appended claims and that all modifications that come within the meaning and range of equivalency of the claims are intended to be included herein.

I claim:

1. A method of preparing the surface of a single metal crystal for optimal electron emission utilized in an alkalimetal plasma converter comprising the steps of:

(A) Slicing the crystal to present a plane having the highest atomic density (B) Mechanically abrading the surface with an abrasive slurry (C) subjecting the abraded surface to heat to establish a crystal temperature within the range of 2250 centigrade to 2600 centigrade under controlled atmospheric conditions (D) Allowing the crystal to gradually dissipate the heat under the same controlled atmospheric conditions. 2. A method of preparing the surface of a single tung- 3 4 sten crystal for optimal electron emission for use in an References Cited alkali metal plasma converter comprising the steps of:

(1) Slicing the crystal on a (110) Miller plane. UNITED STATES PATENTS (2) Mechanically abrading the surface thereof with an 2 541 79 6 19 53 Dakymp1e abrasive slurry. 5 (3) Heating the crystal to a minimum temperature of JOHN CAMPBELL, Primary 2250 C. by means of electron bombardment and maintaining said temperature for a period of not less P. M. COHEN, Assistant Examiner. than one hour or greater than ten hours in an atmospheric pressure of less than 10- torr. 10 US. Cl. X.R. (4) Cooling the crystal under the same controlled at- 29-557, 583

mospheric conditions. 

